Recently, a nonaqueous electrolyte secondary battery such as a lithium-ion secondary battery has been actively researched and developed as a battery having high-energy density. The nonaqueous electrolyte battery is expected to be used as a power source for hybrid vehicles, electric cars, an uninterruptible power supply for base stations for portable telephone, or the like. For this, the nonaqueous electrolyte battery is desired to have other characteristics such as rapid charge-and-discharge characteristics and long-term reliability. For example, a nonaqueous electrolyte battery enabling rapid charge and discharge not only remarkably shortens the charging time but also makes it possible to improve performances of the motive force of a hybrid vehicle or the like and to efficiently recover the regenerative energy.
In order to enable rapid charge and discharge, it is necessary that electrons and lithium ions can migrate rapidly between a positive electrode and a negative electrode.
It is also important that rapid charge and discharge can be safely performed. For example, when a battery using a carbon-based negative electrode repeats rapid charge and discharge, dendrite precipitation of metal lithium occurs on the electrode. There is fear in which the dendrite precipitation leads to internal short circuits, which causes heat generation and fires.
In light of this, a battery using a metal composite oxide in place of a carbonaceous material in a negative electrode has been developed. Particularly, in a battery using a titanium oxide as a negative electrode, rapid charge and discharge can be stably performed. Such a battery also has a longer life than that of the carbon-based negative electrode.
However, the titanium oxide has a higher (nobler) potential relative to metal lithium than that of the carbonaceous material. Further, the titanium oxide has a lower capacity per mass. Thus, the battery using the titanium oxide in the negative electrode has a problem that the energy density is lower.
For example, the potential of the electrode using the titanium oxide is about 1.5 V based on metal lithium, and is higher (nobler) than that of the carbon-based negative electrode. The potential of the titanium oxide is due to the oxidation-reduction reaction between Ti3+ and Ti4+ when lithium is electrochemically absorbed and released, and is therefore limited electrochemically. Furthermore, there is a fact that the rapid charge and discharge of lithium ions can be stably performed at an electrode potential as high as about 1.5 V in the electrode using the titanium oxide. It is therefore substantially difficult to drop the potential of the electrode to improve the energy density.
The other hand, as to the capacity of the battery per unit mass, the theoretical capacity of titanium dioxide having an anatase structure is about 165 mAh/g, and the theoretical capacity of a lithium-titanium composite oxide such as Li4Ti5O12 is also about 180 mAh/g. By contrast, the theoretical capacity of a general graphite-based electrode material is 385 mAh/g or more. Thus, the capacity density of the titanium oxide is significantly lower than that of the carbon-based negative electrode. This is due to a reduction in a substantial capacity because there are only a small number of equivalent lithium-absorbing sites in the crystal structure of the titanium oxide, and lithium is easily stabilized in the structure.